There is nothing very accurate about the 1776 drawing. The hill is wrong and the horse is in the wrong position and at the wrong angle. The hand coloured postcard is just a jobbing photographer's attempt to produce a saleable product from a poor image which didn't show the horse clearly - grey on grey. I collect old postcards and have seen many hand coloured ones which embellish what the camera has failed to record adequately.

Only archaeology is likely to resolve the evolution and changes in the cutting of the figure. I have also read somewhere that the figure was fatter and the lines have grown narrower with recutting over an incredibly long period. Two possibiliites strike me

1 The horse has always been very much like we see it and is true to its original design

or

2 The horse evolved over the centuries and gradually became more naturalistic - similar to the 1776 drawing and the 18th-19th century horses. Then when the 19th century interest in all things "Celtic" was fashionable, it was re-shaped using designs from "Celtic" coins ie Romano British. I have recently thought that the present design was just too good to be true.